Sildenafil interferes with cyclic guanine monophosphate, a molecule involved in a signaling pathway that helps regulate the body's circadian rhythm. When the researchers injected hamsters with the drug and woke them up 6 hours earlier than normal by switching on bright lights, the animals adjusted 25% to 50% more quickly than did controls. However, the drug did not achieve the reverse effect when the scientists delayed turning on bright lights to simulate westbound travel.